Geology of the Chenier Plain of Cameron Parish, Southwestern Louisiana

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The Geological Society of America Field Guide 14 2008

Geology of the Chenier Plain of Cameron Parish, southwestern Louisiana

Donald E. Owen* Department of Earth and Space Sciences, Lamar University, POB 10031, Beaumont, Texas 77710, USA

ABSTRACT

The Chenier Plain of southwestern coastal Louisiana is a Holocene strand plain composed of wooded beach ridges (cheniers) and intervening mudfl at grassy wet- lands. The mudfl ats form as prograding tidal fl ats along the open, but low-energy Gulf of Mexico coast; cheniers form from winnowing of sand and shells from the mudfl ats by waves during transgression. Mudfl ats are deposited when a Mississippi River delta lobe is nearby to the east, and cheniers are formed when distributaries switch to a more distant location farther east. All of the cheniers have formed within approximately the past 3000 yr or less and are progressively younger toward the present coastline. Spits are attached to the cheniers at estuaries; they grow westward in response to the dominant longshore currents. Currently, mudfl ats are prograding in Vermillion Parish to the east, while cheniers form in eastern Cameron Parish along with some regressive beach ridge development in western Cameron Parish. This coast is microtidal with low wave energy. A high rate of subsidence as well as sea-level rise characterizes the Chenier Plain, which is subject to increased wave energy and mud transport every year during many cold-front passages and periodic storm surges associated with tropical cyclones of much lower frequency. Major storm surges can inundate the entire Chenier Plain, wreaking havoc on human settlements.

Keywords: Chenier Plain, Louisiana, Holocene, Chenier, mudﬂ at, hurricane, Gulf of Mexico, Mississippi River delta lobe, spits, estuaries, longshore current, Cameron Parish, Vermillion Parish, beach ridge, sea-level rise, microtidal, subsidence, cold front, tropical cyclone, storm surge.

INTRODUCTION Deﬁ nitions

This paper reviews and summarizes the geology of the Che- A chenier plain is a strand plain consisting of long, narrow- nier Plain of southwestern Louisiana, especially its western por- wooded beach ridges (cheniers) and intervening mudﬂ ats with tion in Cameron Parish. Many geologists have contributed to the marsh or swamp vegetation (Otvos and Price, 1979). The word present-day understanding of the development of the upper Holo- chenier is derived from the Cajun French word, chene, meaning cene sediments and the landforms of the Chenier Plain, and much oak, the dominant tree growing on crests of high cheniers in south- of their work is summarized in this paper. western Louisiana, where cheniers were ﬁ rst named by Russell and Howe (1935) (see Figures 1 and 2). A few other examples of

*[email protected]

Owen, D.E., 2008, Geology of the Chenier Plain of Cameron Parish, southwestern Louisiana, in Moore, G., ed.: Geological Society of America Field Guide 14, 2008 Joint Annual Meeting, Houston, Texas, 5–9 October 2008, p. 27–38, doi: 10.1130/2008.ﬂ d014(02). For permission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved.

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28 Owen

cheniers are present on Earth including the extensive mud belt of Suriname. Recently, Taylor et al. (1996, p. 414) suggested that the term chenier complex be used instead of chenier plain for an area with at least two cheniers separated by muddy units.

Early Work

Formation of the Chenier Plain is related to the process of delta switching of the Mississippi River to the east which affects the down-drift area to the west (Kolb and van Lopik, 1958; Fra- zier, 1967). Howe et al. (1935) and Russell and Howe (1935) ﬁ rst interpreted the Chenier Plain to have formed by alternating suspended sediment deposition and wave erosion of sandy mud, leaving the chenier ridges stranded as winnowed sand and shell deposits. This alternation was controlled by the Mississippi River shifting delta lobes from closer to (depositing mud) and farther from (allowing waves to build cheniers) the Chenier Plain, Figure 1. Geologic map of Louisiana locating Chenier Plain. From respectively. After the Chenier Plain sediments were cored, Fisk Penland and Suter (1989, Fig. 1).

Figure 2. Idealized cross section across Chenier Plain with chronostratigraphic interpretations of facies belts. Cheniers become younger from left (inland) to right (seaward). Modiﬁ ed from Penland and Suter (1989, Fig. 18 therein). Downloaded from fieldguides.gsapubs.org on May 18, 2015

Geology of the Chenier Plain 29

(1955) constructed maps of sediment thickness, and Price (1955) more durable, easily reworked shells such as Crassostrea (Taylor described their origin. Gould and McFarlan (1959) and Byrne et et al., 1996, p. 419). al. (1959) presented more detailed studies of the Chenier Plain architecture and age. PROCESSES According to radiocarbon dates of shells, the oldest cheniers (such as Little Chenier) are farthest landward (Fig. 2), and were Chenier Plain construction was described by Hoyt (1969, shown to be nearly 3000 years old (late Holocene); cheniers are p. 301–302) in the following steps: (1) prograding deposition progressively younger closer to the modern coastline (Brannon et of coastal mudfl ats with included sand and shells that is rapid al., 1957; Gould and McFarlan, 1959). Penland and Suter (1989), enough to preclude removal of fi nes; (2) reworking of mudfl at in one of the most comprehensive papers on the Chenier Plain, sediments during shoreline retreat, building a chenier ridge of presented data showing that sea level in southwestern Louisiana sand and shells while fi nes are reworked and transported offshore was 5–6 m lower than present ~3000 yr B.P., and that all the pre- and alongshore (see Fig. 3); and (3) extension of cheniers by the sent cheniers were built after a rapid rise to near present sea level, longshore current into areas not being actively eroded. which was attained at ~2500 yr B.P. All of these workers agreed Growth of spits, which extended the cheniers into bays, was that formation of the Chenier Plain was closely related to avail- described by Gould and McFarlan (1959, p. 268–269). These ability of suspended mud from the lobe- and distributary-shifting spits, which are slightly younger than the chenier to which they Mississippi River. Most early workers tied chenier formation to are attached, are most common near the mouths of the Sabine and switching between major delta complexes such as the Teche and Calcasieu Rivers (Fig. 4), and, to a lesser extent, the Mermen- Lafourche, but Penland and Suter (1989, p. 257) presented evi- tau River. Todd (1968) described a process of dynamic diversion dence of control of chenier formation by switching among indi- of longshore drifting sediment by interaction with tidal currents vidual delta lobes within the Lafourche delta complex during the exiting river and estuary mouths, causing deposition of spits, past 2500 yr or less. Some of the >2500 yr B.P. dates of Gould especially on the eastern updrift side. Ebb-tidal fl ow causes surf and McFarlan (1959) could be too old because of redeposition of suppression and decrease of longshore current velocity, caus- older shells in younger cheniers. However, most of the radiocar- ing deposition of spits on the updrift side. Easterly reversals of bon dates were obtained from fragile Mulinia shells rather than longshore-current direction by wave refraction around entrance

Figure 3. Hoyt’s Chenier Plain process model. From Penland and Suter (1989, their Fig. 2). Downloaded from fieldguides.gsapubs.org on May 18, 2015

30 Owen ed Figure 4. Map of western Chenier Plain with beach ridges-cheniers and upland Prairie Formation (Pleistocene) outcrops (terraces). Note curved spits east of Sabine River mouth and spits east of Sabine River Note curved (Pleistocene) outcrops (terraces). Figure 4. Map of western Chenier Plain with beach ridges-cheniers and upland Prairie Formation Other ridges are cheniers and spits. Modiﬁ are true beach ridges rather than cheniers. mouth. Ridges along coast near Johnsons Bayou (former Sabine distributary) Calcasieu River from Byrne et al. (1959). Downloaded from fieldguides.gsapubs.org on May 18, 2015

Geology of the Chenier Plain 31 shoals have built spits at the west sides of the Sabine and Calcasieu Severe storm surges associated with major hurricanes may over- river mouths against the dominant westerly regional longshore wash active cheniers, ﬂ attening dunes, and producing washover drift (Taylor et al., 1996, p. 420). Westside spits are poorly devel- sand deposits landward into the adjacent marsh. As the shoreface oped on the Mermentau River, but the river has been deﬂ ected and dune parts of cheniers are eroded by waves and longshore 20 km to the west by spit growth on the east side (Fig. 5). currents, they may retrograde onto washover sands. After initial construction, active cheniers may build up sev- Just after completion of this manuscript, an excellent new eral meters higher than sea level by moderate storm surges above paper on the Chenier Plain by McBride et al (2007) was brought to normal sea level and by dune-building between storm surges. my attention. It develops a six-stage geomorphic process-response

Figure 5. Map of eastern Chenier Plain with cheniers, spits, and upland (Prairie terrace). Note curved spits south of Mermentau River east of Grand Chenier. Little Chenier is the oldest dated chenier on the Chenier Plain. Modiﬁ ed from Byrne et al. (1959). Downloaded from fieldguides.gsapubs.org on May 18, 2015

32 Owen

model to describe Chenier-Plain evolution which should be stud- nier cycle, may overlie these units. A more detailed description of ied by anyone interested in the Chenier Plain. the Chenier Plain sediments is given in Penland and Suter (1989; their Table 1). INTERNAL STRATIGRAPHY Variations on the chenier ridge succession exist at locations where spits and beach ridges have developed instead. Spits typi- The internal stratigraphy of the Chenier Plain has been well cally build at entrances to bays, so they rest on bay mud or tidal- described by Byrne et al. (1959), Gould and McFarlan (1959), channel sand deposits and contain spit platform and foreshore Hoyt (1969), and Penland and Suter (1989). Figure 6 illustrates a classic cross section of offshore, nearshore, marsh, and chenier (washover, beach, and dune) lithofacies. Figure 7 illustrates a typical vertical stratigraphic column through a chenier succession of lithofacies. These fi gures are suffi cient for this general paper, however, the reader who seeks more detail on individual beds and sedimentary structures in a chenier that has been exca- vated should see Gremillion and Payne (1977). A complete regressive-transgressive cycle in the Chenier Plain sediments at a chenier ridge location would consist of the stratigraphic units in Table 1. Units 1–2 are deposited during regression and units 3–4 are deposited during transgression by reworking. Unit 1 may be absent because of nondeposition or erosion prior to mudfl at formation. The contact between units 2 and 3 is typically a diastem produced by wave scour. Unit 4 may show paleosol development. An erosional surface formed near the shoreline may truncate Unit 4. Modern marsh sediments, although not part of the che-

Figure 6. Cross section along north-south line from Creole Ridge to Gulf of Mexico shoreface with lithofacies units. Cheniers are older to Figure 7. Idealized stratigraphic model of chenier vertical left. Line of section is from Creole to Gulf of Mexico shown on Figure lithologic succession with depositional environments iden- 5. From Hoyt (1969, Fig. 3). tiﬁ ed. Modiﬁ ed from Penland and Suter (1989, Fig. 7).

TABLE 1. CHENIER STRATIGRAPHIC UNITS IN A REGRESSIVE-TRANSGRESSIVE CYCLE

4. Coastal dune sand with root structures and possible paleosol. Transgressive 3. Washover sand with reworked shallow marine and brackish shell fauna.

2. Mudflat silty mud with marginal shallow marine fauna and ichnofauna. Regressive 1. Offshore mud with normal shallow marine fauna. Downloaded from fieldguides.gsapubs.org on May 18, 2015

Geology of the Chenier Plain 33

sands below the coastal dune sand (Penland and Suter, 1989, their transport is 47,000–76,000 m3/yr to the west, based on shoreline Fig. 7). Most spits are connected to cheniers. erosion calculations, according to the U.S. Army Corps of Engi- True beach ridges, which are regressive in nature and con- neers (1971). The offshore slope is fairly low, averaging 1:125 tain fresh-water marsh sediments rather than marine mudfl at sed- out to the 10 m depth contour line (Taylor et al., 1996, p. 415). iments between them, are developed only in the Johnson Bayou The rate of relative sea-level rise at the Calcasieu Pass tide-gauge area in the westernmost Chenier Plain in Louisiana. They were station is 0.57 cm/yr, based on the period 1942–1988 (Fig. 8), but originally identifi ed as cheniers, but careful examination of the this is only half of the rise at the Grand Isle station on the Mis- vertical succession in excavations by Kaczarowski (1980) sup- sissippi Delta, where subsidence is greater (Penland and Ramsey, ported their beach-ridge origin. The vertical succession in beach 1990, their Table 2). Relative sea-level rise in the delta is 10 times ridges is from shoreface to foreshore to dune deposits (Penland faster than on most of the rest of Earth (Penland and Ramsey, and Suter, 1989, their Fig. 7). As might be expected, these beach 1990, p. 323). ridges are generally younger (<900 yr B.P.) than the cheniers The high rate of subsidence is the dominant factor in shore- inland of them, and these beach ridges are the only signifi cant line erosion on the Louisiana coast, including the Chenier Plain area of prograding sandy/shelly shoreline today (Byrnes et al., coast, in spite of the low energy of waves and tides on this 1995, their Fig. 2). microtidal coast. The Johnsons Bayou beach ridges on the west- Both beach ridges and cheniers have obviously steeper ernmost Louisiana coast between the Sabine River and Ocean slopes on the seaward side than on the landward. Dunes tend to View beach, as mentioned above, are prograding (3.5 m/yr from be higher on modern beach ridges than cheniers. 1883 to 1994) according to Byrnes et al. (1995, p. 116). Accord- ing to Byrnes et al. (1995), the Calcasieu headland coast (Fig. MODERN CONDITIONS 9), which is devoid of cheniers other than the modern beach, from Ocean View beach through Peveto Beach to Calcasieu The Chenier Plain today is in a time of transition—the east- Pass (Fig. 9) is retreating, especially in the western area between ern part in Vermillion Parish is beginning to accumulate prograd- Ocean View beach and Holly Beach, at an average rate of ing mudfl ats at a rapid rate, the central part in eastern Cameron 1.2 m/yr (1883–1994). Retreat has been greater in recent years, Parish is still in the erosional, chenier-forming stage, and the and breakwaters, a revetment to protect the highway, and artifi - western part in western Cameron Parish is experiencing a period cial beach nourishment have been put in place to contest coastal of slow, regressive beach-ridge building. All of this is occurring erosion in this area. The eastern area between Holly Beach and on a microtidal, low–wave-energy coast. According to Byrnes et the Calcasieu River has been eroding at approximately the same al. (1995, p. 113), tides range from 0.6 to 0.8 m (mean to spring); rate as the chenier coast to the west. The area near Calcasieu Pass, locally generated wind waves from the south and southeast domi- especially on the east side of the jetties, has been developing a nate 18% and 22% of the time, respectively; and breaking wave prograding mudfl at in the wave shadow of the shoals off Calca- heights average 0.5 m with a 5-second period. Net longshore sand sieu Pass. This section of the coast has very low wave energy at

Cameron 1942 - 1988 60

30 Mean (cm)

0 1940 1950 1960 1970 1980 1990 Year

Figure 8. Water level time-series graph for Calcasieu Pass tide-gauge station near Cameron, Louisiana for period of 1942–1988. Mean relative sea-level rise is 0.57 cm/yr. Adapted from Penland and Ramsey (1990, their Fig. 9). Downloaded from fieldguides.gsapubs.org on May 18, 2015

34 Owen

Figure 9. Landforms of the Calcasieu headland area. Ridges along coast near Johnsons Bayou are true beach ridges. Other ridges to north and near Cameron are cheniers and spits. From Penland and Suter (1989, their Fig. 12).

the shoreline. Some of the mudfl at was eroded by the storm surge shoreface, where it is stranded on these new mudfl ats. The sus- of Hurricane Rita during September, 2006, leaving shell-armored pended fi ne mud in the nearshore zone attenuates wave energy, mud balls on the remaining mudfl at. A similar hurricane, Audrey, facilitating mud deposition at the very low-energy shoreline in 1957, affected the same area. Chenier Plain response to Hur- (Wells, 1986, p. 428). ricane Bonnie, a category 1 storm during 1986, was described by Nakashima (1989). CONCLUSIONS Modern accretion of mudfl ats west of Vermillion Bay in the eastern Chenier Plain of Vermillion Parish began during ca. 1950 The Chenier Plain is a strand plain consisting of long, A.D. (Morgan et al., 1953), coincident with the development of narrow-wooded beach ridges (cheniers) and intervening mud- the new subaqueous delta and its discharge plume in Atchafa- fl ats with marsh or swamp vegetation. It formed by alternating laya Bay, just east of Vermillion Bay, the fi rst pulse of sediment suspended sediment deposition and wave erosion of sandy mud, accumulation in this area after 1000 yr (Wells and Kemp, 1981, leaving the chenier ridges stranded as winnowed bed load sand p. 409). The Atchafalaya mud stream (Figs. 10 and 11) has been and shell deposits as the Mississippi shifted delta lobes from building mudfl ats at the high rate of ~50 m/yr (Huh et al., 2001, closer to and farther from the Chenier Plain, respectively. The p. 79). Huh et al. (2001; their Fig. 8) illustrated a new mudfl at Chenier Plain today is in a time of transition—the eastern part is (Fig. 12) that had attained a width of 770 m (510 m of which beginning to accumulate prograding mudfl ats at a rapid rate, the is subaerial) in 11 yr (1987–1998). Roberts et al. (2002, p. 850) central part is still in the erosional, chenier-forming stage, and the also concluded that winter cold-front passages (20–30/yr) and western part is experiencing a period of slow regressive beach- tropical storms every few years result in a water level set-up and ridge building. All of this is occurring on a microtidal, low–wave- shore-normal transport of mud from the nearshore shelf onto the energy coast affected by frequent winter cold-front passages and Downloaded from fieldguides.gsapubs.org on May 18, 2015

Geology of the Chenier Plain 35 Figure 10. Map of Atchafalaya discharge plume and mud stream that deposits mudﬂ ats on eastern Chenier Plain. Note zone of coastal erosion western From Huh et plume and mud stream that deposits mudﬂ discharge Atchafalaya Figure 10. Map of al. (2001, their Fig. 2). Downloaded from fieldguides.gsapubs.org on May 18, 2015

36 Owen Atchafalaya discharge discharge Atchafalaya ected solar radiation image of Louisiana coastal area, calibrated to suspended sediment content of water, 21 April 1990. Note 21 ected solar radiation image of Louisiana coastal area, calibrated to suspended sediment content water, Figure 11. NOAA satellite AVHRR reﬂ AVHRR satellite Figure 11. NOAA Depths in meters. From Huh et al. (2001, their Fig. 5a). This is the normal modern condition. Bay and hugging coast westward. Atchafalaya colors) coming out of plume (orange-to-yellow Downloaded from fieldguides.gsapubs.org on May 18, 2015

Geology of the Chenier Plain 37

Figure 12. Aerial photographs of triple canal site (c) west of Atchafalaya Bay, Vermillion Parish, on eastern Chenier Plain coast. Upper photo (A): 27 January 1987. Lower photo (B): 4 April 1998. On (B), note plant-covered mudﬂ at (red) extending 510 m from previous shoreline and plant-free mud (light blue) on shoreface between 510 and 770 m. From Huh et al. (2001, their Fig. 8). Downloaded from fieldguides.gsapubs.org on May 18, 2015

38 Owen

Kolb, C.R., and Van Lopik, J.R., 1958, Geology of the Mississippi deltaic plain: occasional tropical storms that infl uence coastal processes. Major U.S. Army Corps of Engineers, Waterways Experiment Station, Technical tropical cyclone landfalls near the west end of the Chenier Plain Report 2, 482 p. can produce storm surges that inundate the entire Chenier Plain, McBride, R.A, Taylor, M.J., and Byrnes, M.R., 2007, Coastal morphodynamics and Chenier-Plain evolution in southwestern Louisiana, USA: A geomor- wreaking havoc on human settlements. phic model: Geomorphology, v. 88, p. 367–422. Morgan, J.P., Van Lopik, J.R., and Nichols, L.G., 1953, Occurrence and devel- ACKNOWLEDGMENTS opment of mudfl ats along the western Louisiana coast: Louisiana State University Coastal Studies Institute, Technical Report 2, 34 p. Nakashima, L.D., 1989, Shoreline responses to Hurricane Bonnie in southwest- Thanks to Richard A. 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Field Guides

Geology of the Chenier Plain of Cameron Parish, southwestern Louisiana

Donald E. Owen

Field Guides 2008;14;27-38 doi: 10.1130/2008.fld014(02)

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Notes